Latency and Latency-Compensation

Latency
is a system's reaction time to a given stimulus. There are many factors that
contribute to the total latency of a system. In order to achieve exact time
synchronization all sources of latency need to be taken into account and
compensated for.

Sources of Latency

Sound propagation through the air

Since sound is a mechanical perturbation in a fluid, it travels at
comparatively slow speed
of about 340 m/s. As a consequence, your acoustic guitar or piano has a
latency of about 1–2 ms, due to the propagation time of the sound
between your instrument and your ear.

Digital-to-Analog and Analog-to-Digital conversion

Electric signals travel quite fast (on the order of the speed of light),
so their propagation time is negligible in this context. But the conversions
between the analog and digital domain take a comparatively long time to perform,
so their contribution to the total latency may be considerable on
otherwise very low-latency systems. Conversion delay is usually below 1 ms.

Digital Signal Processing

Digital processors tend to process audio in chunks, and the size of that chunk
depends on the needs of the algorithm and performance/cost considerations.
This is usually the main cause of latency when you use a computer and one you
can try to predict and optimize.

Computer I/O Architecture

A computer is a general purpose processor, not a digital audio processor.
This means our audio data has to jump a lot of fences in its path from the
outside to the CPU and back, contending in the process with some other parts
of the system vying for the same resources (CPU time, bus bandwidth, etc.)

The Latency chain

Figure: Latency chain.
The numbers are an example for a typical PC. With professional gear and an
optimized system the total roundtrip latency is usually lower. The important
point is that latency is always additive and a sum of many independent factors.

Processing latency is usually divided into capture latency (the time
it takes for the digitized audio to be available for digital processing, usually
one audio period), and playback latency (the time it takes for
In practice, the combination of both matters. It is called roundtrip
latency: the time necessary for a certain audio event to be captured,
processed and played back.

It is important to note that processing latency in a jackd is a matter of
choice. It can be lowered within the limits imposed by the hardware (audio
device, CPU and bus speed) and audio driver. Lower latencies increase the
load on the system because it needs to process the audio in smaller chunks
which arrive much more frequently. The lower the latency, the more likely
the system will fail to meet its processing deadline and the dreaded
xrun (short for buffer over- or under-run) will make its
appearance more often, leaving its merry trail of clicks, pops and crackles.

The digital I/O latency is usually negligible for integrated or
PCI audio devices, but
for USB or FireWire interfaces the bus clocking and buffering can add some
milliseconds.

Low Latency usecases

Low latency is not always a feature you want to have. It
comes with a couple of drawbacks: the most prominent is increased power
consumption because the CPU needs to process many small chunks of audio data,
it is constantly active and can not enter power-saving mode (think fan-noise).
Since each application that is part of the signal chain must run in every
audio cycle, low-latency systems will undergocontext switches
between applications more often, which incur a significant overhead.
This results in a much higher system load and an increased chance of xruns.

For a few applications, low latency is critical:

Playing virtual instruments

A large delay between the pressing of the keys and the sound the instrument
produces will throw-off the timing of most instrumentalists (save church
organists, whom we believe to be awesome latency-compensation organic systems.)

Software audio monitoring

If a singer is hearing her own voice through two different paths, her head
bones and headphones, even small latencies can be very disturbing and
manifest as a tinny, irritating sound.

Live effects

Low latency is important when using the computer as an effect rack for
inline effects such as compression or EQ. For reverbs, slightly higher
latency might be tolerable, if the direct sound is not routed through the
computer.

Live mixing

Some sound engineers use a computer for mixing live performances.
Basically that is a combination of the above: monitoring on stage,
effects processing and EQ.

In many other cases, such as playback, recording, overdubbing, mixing,
mastering, etc. latency is not important, since it can easily be
compensated for.
To explain that statement: During mixing or mastering you don't care
if it takes 10ms or 100ms between the instant you press the play button
and sound coming from the speaker. The same is true when recording with a count in.

Latency compensation

During tracking it is important that the sound that is currently being
played back is internally aligned with the sound that is being recorded.

This is where latency-compensation comes into play. There are two ways to
compensate for latency in a DAW, read-ahead and
write-behind. The DAW starts playing a bit early (relative to
the playhead), so that when the sound arrives at the speakers a short time
later, it is exactly aligned with the material that is being recorded.
Since we know that play-back has latency, the incoming audio can be delayed
by the same amount to line things up again.

As you may see, the second approach is prone to various implementation
issues regarding timecode and transport synchronization. Ardour uses read-ahead
to compensate for latency. The time displayed in the Ardour clock corresponds
to the audio-signal that you hear on the speakers (and is not where Ardour
reads files from disk).

As a side note, this is also one of the reasons why many projects start at
timecode 01:00:00:00. When compensating for output latency the
DAW will need to read data from before the start of the session, so that the
audio arrives in time at the output when the timecode hits 01:00:00:00.
Ardour3 does handle the case of 00:00:00:00 properly but not all
systems/software/hardware that you may inter-operate with may behave the same.

Latency Compensation And Clock Sync

To achieve sample accurate timecode synchronization, the latency introduced
by the audio setup needs to be known and compensated for.

In order to compensate for latency, JACK or JACK applications need to know
exactly how long a certain signal needs to be read-ahead or delayed:

Figure: Jack Latency Compensation.

In the figure above, clients A and B need to be able to answer the following
two questions:

How long has it been since the data read from port Ai or Bi arrived at the
edge of the JACK graph (capture)?

How long will it be until the data writen to port Ao or Bo arrives at the
edge of the JACK graph (playback)?

JACK features an API
that allows applications to determine the answers to above questions.
However JACK can not know about the additional latency that is introduced
by the computer architecture, operating system and soundcard. These values
can be specified by the JACK command line parameters -I
and -O and vary from system
to system but are constant on each. On a general purpose computer system
the only way to accurately learn about the total (additional) latency is to
measure it.

Calibrating JACK Latency

Linux DSP guru Fons Adriaensen wrote a tool called jack_delay
to accurately measure the roundtrip latency of a closed loop audio chain,
with sub-sample accuracy. JACK itself includes a variant of this tool
called jack_iodelay.

Jack_iodelay allows you to measure the total latency of the system,
subtracts the known latency of JACK itself and suggests values for
jackd's audio-backend parameters.

jack_[io]delay works by emitting some rather annoying tones, capturing
them again after a round trip through the whole chain, and measuring the
difference in phase so it can estimate with great accuracy the time taken.

You can close the loop in a number of ways:

Putting a speaker close to a microphone. This is rarely done, as air
propagation latency is well known so there is no need to measure it.

Connecting the output of your audio interface to its input using a
patch cable. This can be an analog or a digital loop, depending on
the nature of the input/output you use. A digital loop will not factor
in the AD/DA
converter latency.

Once you have closed the loop you have to:

Launch jackd with the configuration you want to test.

Launch jack_delay on the commandline.

Make the appropriate connections between your jack ports so the loop is closed.